1. Field of the Invention
This invention pertains to a pole mounting type of electrical distribution transformer such as used for converting a 13,200 volt A.C. input to a 240/120 volt, single phase A.C. output for domestic and other usage that will permit hand hole entrance for maintenance and repair work and also provide total safety protection when used with a suitable current limiting fuse from the standpoint of avoiding blow-out of the top end wall under a build-up of high, fault-produced, internal fluid pressure. A phase of the invention deals with an improved transformer casing that has a pre-shaped wall construction based on its being subjected to relatively high positive internal fluid pressure, and that will have a combined hand hole cover and a simple and highly effective pressure relief means capable of efficient operation between lower cut-off to materially higher internal fluid pressures.
2. Description of the Prior Art
In recent years there has been a growing problem in the construction of overhead pole type distribution transformers from the standpoint of the blowing-off of upper cover end walls under internal fluid pressures developed by over-pressure phenomena, such as caused by electric arcing and other electrical faulting. Such transformers are mounted near the top of utility poles in residential areas, thus they tend to menace both persons and properties, since oil or other content may be ejected to fall as a hot and possibly fiery rain on anything or anyone in the immediate vicinity. Although so-called bleeder or light duty valve means have been mounted on the side of transformer tanks or housings for normal relatively low pressure venting, such means has been found totally unsatisfactory from the standpoint of a sudden and particularly high fluid pressure generation within the transformer such as may occur due to a fault of electrical equipment therein.
A conventional end wall cover which is retained in position over the upper edges of the side wall of a tank or housing by a take-up clamping chime ring similar to those used on dry bulk storage drums, represents a type that is most likely to be dislodged with considerable force under such conditions. Such covers for pole type transformers are made removable, since there are no other openings to permit access to the inside of the housing for modifying internal connections, operating internal tap changing switches and, in general, for easy maintenance.
Another type of end wall cover is removably mounted over the upper edge of the side wall of a transformer housing by means of a centrally projecting lug bolt whose inner end is threaded into an integrally secured cross piece within the housing and which serves to draw up the cover to a tightened relation on the circumferential edge of the housing. Both of these constructions entail risk from the standpoint of cover blow-off under high or sudden internal fluid pressure applications, such as occur from a failure to relieve quickly arising pressure that develops rapidly within the casing due to a fault which may result from the formation of an electrical arc under the oil dielectric. Such an arc may be caused by insulation failure or the fusing-apart of wires within or near the transformer windings, such as may occur under adverse operating conditions or as a result of some defect. Cover end wall blowing may be due to a very rapid internal pressure rise that can lift the oil column and slam it against the cover, simultaneously producing a force in the opposite direction against the bottom of the tank or housing and as well as forces against its sides. A second type has a fairly slow rise which gradually builds up in the transformer top and results in a blown cover and/or a deformed tank. The fault may or may not cause vaporization of the oil within the air space above the normal dielectric level therein.
As above indicated, side mounted relief devices have been provided to act as safety valves against normal pressure build-ups, such as due to loading and overloading of transformers. These valves are relatively small and inadequate to effectively relieve rapid build-up of pressure such as may occur as when internal arcing is present. There has been a need for an improved type of transformer tank or housing construction which can be relatively inexpensively produced and which will meet the problem involved in previous constructions, particularly from the standpoint of preventing high pressure dislodgement of the upper end wall and, in general, pressure deformation of a transformer tank or housing and, at the same time, which will permit entry to the inside of the casing for maintenance and repair of the electrical equipment therewithin.